This Special Issue is devoted to the illustration and discussion of three key demographic variables (sex/gender, race/ethnicity, and socioeconomic status) that have been shown to moderate associations between psychophysiological processes and behavior. The introduction to the issue discusses the role of phenotypic plasticity in the emergence of different neural processes that achieve the same behavioral outcome, with emphasis on how these relatively stable developmental contexts affect brain/behavior associations without necessarily resulting in difference in behavior. These findings have profound significance for the implications of generalization and call into question the presumption that diverse samples produce an average result that is appropriately reflective of the individuals themselves. Increasing diversity within psychophysiological research is critical in elucidating mechanisms by which the human brain can accomplish cognitive and affective behaviors. This article further examines the logistical and ethical challenges faced in achieving this goal.